This invention pertains to a supported silver catalyst, its manufacture, and its use in the production of an alkylene oxide, particularly, ethylene oxide, directly from oxygen and an olefin, such as ethylene.
Alkylene oxides are known for a multiplicity of utilities. Ethylene oxide, for example, is used to produce ethylene glycol, which is used in preparing polyester fibers and resins, nonionic surfactants, glycol ethers, ethanolamines, and polyethylene polyether polyols. Propylene oxide is used to produce propylene glycol and polypropylene polyether polyols, which are used in polyurethane polymer applications.
The manufacture of ethylene oxide by the direct reaction of ethylene with oxygen or an oxygen-containing gas in the presence of a silver catalyst is an old and well developed art. An outline of the history of direct ethylene oxidation can be found in U.S. Pat. No. 4,916,243. This patent, more particularly, describes a catalyst comprising silver deposited on an alpha-alumina macroporous support, further comprising cesium and at least one other alkali metal selected from the group consisting of lithium, sodium, potassium, and rubidium, such that the combination of cesium and other alkali metals exhibits a synergistic promoting effect on the oxidation process.
A supported silver catalyst for alkylene oxide manufacture should have acceptable activity, efficiency, and stability. The “activity” of a catalyst can be quantified in a number of ways, one being the mole percent of alkylene oxide contained in the outlet stream of the reactor relative to that in the inlet stream (the mole percent of alkylene oxide in the inlet stream typically, but not necessarily, approaches zero percent) while the reactor temperature is maintained substantially constant; and another being the temperature required to maintain a given rate of alkylene oxide production. In many instances, activity is measured over a period of time in terms of the mole percent of alkylene oxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant mole percent of alkylene oxide, such as ethylene oxide. The “efficiency” of the oxidation, which is synonymous with “selectivity,” refers to the total amount, in molar percent, of converted or reacted olefin that forms a particular product. For example, the “selectivity to alkylene oxide” refers to the percentage on a molar basis of converted or reacted olefin that forms alkylene oxide. One measure of the useful life of a catalyst is the length of time that reactants can be passed through the reaction system during which time acceptable productivity is obtained in light of all relevant factors. “Deactivation”, as used herein, refers to a permanent loss of activity and/or efficiency, that is, a decrease in activity and/or efficiency that cannot be recovered. Generally, deactivation tends to proceed more rapidly when higher reactor temperatures are employed. The “stability” of a catalyst is inversely proportional to the rate of deactivation. Lower rates of deactivation are generally desirable.
In recent years, improvements in activity, efficiency, and stability of alkylene oxide catalysts have been achieved with the use of carriers comprising high-purity alpha-alumina of greater than 80 weight percent compositional purity. For example, U.S. Pat. Nos. 4,994,588 and 4,994,587 and references therein disclose carriers of high-purity alpha-alumina and methods of making them. The carriers and catalysts derived from these carriers typically do not contain binders, such as certain clays. Binders tend to introduce quantities of extraneous metals, particularly alkali metal-containing species, which may influence the performance of the catalyst prepared on those carriers. Although high purity alpha-alumina is desirable for the control of alkali metal content, other modifiers may be added to the carrier in order to improve catalyst performance. For example, WO-A1-2005/039757 discloses high-purity alpha-alumina carriers containing zirconium silicate (zircon), and EP1354626 and U.S. Pat. No. 5,145,824 describe carriers prepared with various modifier components.
The above-described references are silent regarding the effects of reactor upsets on catalyst performance. For the purposes of this invention, the term “reactor upset” shall refer to an interruption in the alkylene oxide process that may occur, for example, as a result of mechanical or electrical failure in process equipment, or shut-down due to loss of process control or external influences (e.g., detrimental weather conditions), or interruption or stoppage resulting for any reason other than normal catalyst aging. Reactor upsets may vary in duration from about a few minutes to about several months. Reactor upsets frequently present recovery problems with the catalyst. Catalyst activity and/or efficiency may not recover to pre-upset levels as quickly as desired. In fact, several weeks may elapse before catalyst activity and/or efficiency are fully recovered. Worse still, the catalyst may never recover to pre-upset levels of activity and/or efficiency, but rather may settle into lower activity and/or efficiency levels. Each reactor upset results in lost productivity; but often the effect is permanent, rather than temporary, because the catalyst fails to recover its former activity and/or efficiency.